Open Access
Add to BibSonomyAbstract
We generated sub-kilowatt peak-power and 6-ps duration 390-nm optical pulses via the fourth harmonic generation of amplified optical output from a gain-switched 1.55-μm laser diode. We obtained a power-conversion-efficiency of 12% from 1.55-μm to 390-nm light, and subsequently applied the ultraviolet pulses to time-resolved spectroscopy of blue-violet luminescent materials, including a Coumarine dye solution and nitride semiconductor materials using single-photon and two-photon excitation schemes.
©2010 Optical Society of America
1. Introduction
Ultrashort optical pulses in the near-ultraviolet (NUV) spectral region are becoming more important for many applications, including the time-resolved luminescence spectroscopy of wide-gap semiconductor materials, biological lifetime-imaging, and high-density optical data storage. Mode-locked solid-state lasers (MLSLs) are typically used as optical pulse sources for generating NUV optical pulses via the second-, third-, or fourth-harmonic generation [1]. These lasers, however, have several disadvantages: they are large and costly, they require maintenance, and they are not easy to synchronize to electronic signals. Recently, NUV pulse generation via the fourth-harmonic generation (FHG) of amplified optical pulses from a mode-locked fiber laser (MLFL) has been demonstrated, with a conversion-efficiency of 5.5% from the fundamental wavelength [2]. Thus, this approach can provide a compact NUV optical pulse source. However, with a conventional MLFL, it is still not easy to achieve electronic synchronization, which is required for the control of operational timing with many measurement instruments. Our recent study revealed that InGaN laser diodes driven by nanosecond electric pulses could produce 10-ps optical pulses with a peak power of over 12 W [3]. However, this still needs further work to realize peak powers of over 100 W. In this paper, we report the generation of electrically synchronized NUV optical pulses via the FHG of amplified optical pulses generated from a gain-switched 1.55-μm laser diode. The FHG power conversion efficiency from the fundamental wavelength was over 10%, and the FHG optical pulse peak power reached 200 W. We show results of time-resolved spectroscopy of blue fluorescent materials, including a Coumarine dye solution and a InGaN quantum-well structure, using FHG optical pulses. Two-photon excited time-resolved spectroscopy is also demonstrated for a GaN crystal, which emits ultraviolet luminescence.
2. High-peak-power 1.55-μm picosecond optical pulse source and high-efficiency second-harmonic generation
Our experimental setup is shown in Fig. 1 . We generated optical pulses by gain-switching an InGaAsP multi-quantum well (MQW) distributed-feedback-Bragg structure laser diode (DFB-LD), mainly at a repetition rate of 10 MHz. The operation wavelength was set to 1548 nm using temperature control. We used electric pulses of 150–200 ps duration and a voltage-amplitude of 5 V to excite the DFB-LD. Using this method, we directly generated optical pulses of 5–10 ps duration. We used a 10-mW saturation-power erbium-doped fiber amplifier (EDFA) as a pre-amplifier for the optical pulses. After the pre-amplification, the optical pulses were filtered out with a 1-nm-band-width optical filter to remove optical noise caused by spontaneous emission. As the main optical amplifier, we used a low-nonlinear-effect Ce-codoped EDFA to avoid spectral distortions due to self-phase-modulation (SPM) inside the EDFA [4]. This main EDFA was specially designed for this experiment, with a core-diameter of 10 μm and an active-fiber length of approximately 0.9 m. The maximum average output power was about 100 mW. Therefore, if the pulse repetition rate and width were set to 10 MHz and 5 ps, respectively, 2-kW peak-power optical pulses could be generated. However, at 2-kW peak power, we observed a notable spectral broadening due to SPM, and so we used the DFB-LD to generate 7-ps optical pulses. After the main optical amplifier, frequency-doubled 774-nm optical pulses were obtained by second-harmonic generation (SHG) in a 5-mm-long bulk-type periodically poled MgO-doped LiNbO3 (PPMgLN) crystal [5–7]. SHG power and conversion efficiency dependences on the fundamental-wavelength optical power are shown in Fig. 2 ; the maximum SHG conversion efficiency was 55%. As the peak power of the SHG pulses exceeds a kilowatt, efficient supercontinuum (SC) light generation is possible with a photonic crystal fiber [8]. However, SC light generation in the violet and ultraviolet spectral regions still needs a specially-designed photonic crystal fiber [9], and the extracted power of the SC light in these wavelength regions is not enough for the purpose of high-density excitation of materials. Therefore, FHG can be a good alternative method for the generation of NUV optical pulses.
Fig. 1 Schematic of the configuration for generating 387-nm optical pulses via the FHG of amplified 1.55 μm optical pulses from a gain-switched laser diode. PPMgLN: periodically poled MgO-doped LiNbO3; PPKTP: periodically poled KTiPO4; BPF: band pass optical filter; EDFA: Er-doped fiber amplifier.
3. Fourth harmonic generation
In order to generate NUV optical pulses by FHG of an amplified 1548-nm output, we initially used a bulk β-BaB2O4 (BBO) crystal to test for frequency doubling of 774-nm optical pulses. However, the maximum conversion efficiency of the 1548-nm output was only a few percent, and the peak-power did not exceed 50 W.
Instead, we prepared a bulk-type periodically poled KTiPO4 (PPKTP) crystal in order to obtain a higher FHG conversion efficiency. The PPKTP crystal (provided by Raicol Crystal Ltd.) was 5 mm long, 2 mm wide, and 1 mm thick. The polarization-inversion period of the crystal was 2.83 μm to realize a first-order quasi-phase-matched structure [10], and the phase-matching condition for FHG was obtained at a temperature of around 45°C. The 774-nm SHG output was collimated, and a 1-mm diameter beam was focused into the PPKTP crystal using a 50-mm focal-length lens. The transverse mode of the FHG pulses was confirmed to be Gaussian-like single mode and there were not notable beam distortions in a beam-profile measurement. Figure 3 shows the dependences of the FHG output power and conversion efficiency on the 1548-nm optical power; a conversion efficiency of 12% was obtained. The conversion efficiency from SHG to FHG optical power was over 20%.
Fig. 3 (a) FHG power and conversion efficiency dependences on the 1548-nm optical power, (b) optical spectrum of the FHG optical pulses.
We measured the FHG optical-pulse using an ultrafast streak camera (Hamamatu Photonics model C5680), and found a pulse duration of 6 ps and a maximum peak power of 200 W (optical pulse energy was 1.2 nJ). Optical spectrum measurements revealed that the spectral width was 0.04 nm (frequency width was 80 GHz) and that the FHG optical pulses were almost Fourier-transform limited. The FHG optical pulses were broader when PPKTP crystal was used (6 ps) than with the BBO crystal (4 ps). This is due to the phase-matching bandwidth’s being limited by a combination of the crystal length and the laser beam focusing condition in our experiment, which was necessary to obtain an optimal averaged FHG power.
4. Time-resolved spectroscopy of blue-violet fluorescent materials
The NUV optical pulses had sufficient conversion efficiency for spectroscopy applications, and we carried out time-resolved photoluminescence measurements on a selection of blue-violet fluorescent materials using a combination of the ultrafast streak camera and a monochromator (Hamamatsu Photonics C5094).
Figure 4 shows spectrally resolved fluorescence temporal decay for a Coumarine 480 dye ethanol solution (concentration: 10−4 mol/L). The shorter wavelength components rapidly decay in a sub-nanosecond timescale, whereas longer wavelength components decay much more slowly. This is because the dye molecules that are excited to higher vibrational energy states rapidly relax to lower energy states, which have lifetimes of a few nanoseconds.
It should be noted that the dynamic range of the present streak camera measurement is approximately two orders of magnitude, and thus the very low intensity level portions in the decay curve traces, including rising edges, do not give accurate relative-level indications (and this is also same in Figs. 5 and 6 ).
Fig. 5 (a) Spectrally-resolved streak-camera image of the PL from a InGaN/GaN SQW, (b) the integrated PL spectrum of the streak-camera image shown in (a), (c) PL decay curves extracted from the streak-camera image at 425 nm, 440 nm, and 455 nm.
Fig. 6 (a) Spectrally-resolved streak-camera image of PL from GaN crystal, (b) PL decay curve extracted from the streak-camera image at 373 nm.
The spectrally resolved streak camera traces shown in Fig. 5 are for an InGaN/GaN single-quantum-well (SQW) structure grown on a (0001) sapphire substrate. The time-resolved PL shows a dependence on the wavelength that is qualitatively similar to that in Fig. 4, which indicates that photo-excited carrier distributions rapidly relax to quasi-equilibrium states before radiative recombination.
The above time-resolved PL measurements are of single-photon excitation process, but sub-kilowatt peak power optical pulses can be also used for multi-photon excitation. We performed a two-photon-excitation time-resolved PL measurement of GaN crystal grown by halide vapor phase epitaxy. Using a tightly focused FHG, we observed the PL decay curves for the GaN crystal shown in Fig. 6, although the PL intensity was not very strong. The PL decay shows a single exponential decay with a 2.2-ns time-constant, which is attributed to non-radiative carrier recombination through carrier trapping by crystal-defects in the non-doped GaN crystal.
5. Conclusions
In summary, we obtained electrically synchronized NUV picosecond optical pulses via the FHG of amplified 1.55-μm optical pulses from a gain-switched 1.55-μm laser diode. The maximum FHG power conversion efficiency was 12%, and the maximum FHG optical pulse peak power was approximately 200 W. These optical pulses were applied to time-resolved spectroscopy of blue-violet materials, including a Coumarine dye solution and an InGaN single-quantum-well structure. Additionally, two-photon-excited time-resolved spectroscopy of a GaN crystal was demonstrated. These results indicate that the NUV picosecond optical pulse source based on a laser diode would have potential for analysis of materials, including biological specimens, that exhibit absorbance or fluorescence in the blue, violet, and ultraviolet spectral regions. It is expected that further increase in the peak-power (up to 1 kW) will enable the optically pumped ultrashort-pulse laser action of various laser materials, including semiconductor and organic crystals.
Acknowledgments
The authors thank to H. Akiyama and S. Sato for their helpful discussions. The authors also acknowledge the support of T. Kobayashi, M. Sugou, S. Oku, and T. Miyajima for the preparation of InGaN and GaN semiconductor specimens. The present research was supported in part by Core Research for Evolutional Science and Technology (CREST) from JST, Japan, and in part by a Grant-in-Aid for Scientific Research on Innovative Area “Optical Science of Dynamically Correlated Electrons” (Grant No. 20104004) from MEXT, Japan.
References and links
1. M. Ghotbi and M. Ebrahim-Zadeh, “990 mW average power, 52% efficient, high-repetition-rate picosecond-pulse generation in the blue with BiB3O6.,” Opt. Lett. 30(24), 3395–3397 (2005). [CrossRef]
2. O. Kuzucu, F. N. G. Wong, D. E. Zelmon, S. M. Hegde, T. D. Roberts, and P. Battle, “Generation of 250 mW narrowband pulsed ultraviolet light by frequency quadrupling of an amplified erbium-doped fiber laser,” Opt. Lett. 32(10), 1290–1292 (2007). [CrossRef] [PubMed]
3. S. Kono, T. Oki, T. Miyajima, M. Ikeda, and H. Yokoyama, “12 W peak-power 10 ps duration optical pulse generation by gain switching of a single-transverse-mode GaInN blue laser diode,” Appl. Phys. Lett. 93(13), 131113 (2008). [CrossRef]
4. Y. Kubota, T. Teshima, N. Nishimura, S. Kanto, S. Sakaguchi, Y. Zhicong Meng, Nakata, and T. Okada, “Novel Er and Ce codoped fluoride fiber amplifier for low-noise and high-efficient operation with 980-nm pumping,” IEEE Photon. Technol. Lett. 15(4), 525–527 (2003). [CrossRef]
5. H. Taniguchi, M. Kotoh, S. Maeda, K. Abe, and O. Tohyama, “Development of wavelength converter based on quasi-phase-matched PPMgLN waveguide,” Mitsubishi Cable Ind. Rev. 99, 29–34 (2002) (JIHOU).
6. H. Yokoyama, M. Shirane, Y. Sasaki, H. Ito, and H. Taniguchi, “Supercontinuum generation in 800-nm wavelength region with semiconductor laser pulses,” presented at Nonlinear Optics 2004, (Waikoloa, Hawaii, Aug. 2004), ThB3.
7. H. Yokoyama, H. C. Guo, T. Yoda, K. Takashima, K. Sato, H. Taniguchi, and H. Ito, “Two-photon bioimaging with picosecond optical pulses from a semiconductor laser,” Opt. Express 14(8), 3467–3471 (2006). [CrossRef] [PubMed]
8. H. Yokoyama, H. Tsubokawa, H. C. Guo, J. Shikata, K. Sato, K. Takashima, K. Kashiwagi, N. Saito, H. Taniguchi, and H. Ito, “Two-photon bioimaging utilizing supercontinuum light generated by a high-peak-power picosecond semiconductor laser source,” J. Biomed. Opt. 12(5), 054019 (2007). [CrossRef] [PubMed]
9. J. M. Stone and J. C. Knight, “Visibly “white” light generation in uniform photonic crystal fiber using a microchip laser,” Opt. Express 16(4), 2670–2675 (2008). [CrossRef] [PubMed]
10. S. Wang, V. Pasiskevicius, F. Laurell, and H. Karlsson, “Ultraviolet generation by first-order frequency doubling in periodically poled KTiOPO4.,” Opt. Lett. 23(24), 1883–1885 (1998). [CrossRef]
